Chlorinated poly(vinyl chloride) resin (hereafter "CPVC") has gained in popularity for use in extruded pipe, cable jacketing and structural components for buildings, mainly because of its stability. However, such popularity is tempered by the fact that commercially available CPVC having a chlorine content above 69% is difficult to process. Such CPVC is commercially prepared mainly by the chlorination of an aqueous suspension of microporous PVC macrogranules ("granules"for brevity) in the presence of UV (ultraviolet) light radiation (referred to as the "photo-slurry process"). In some processes the UV-light may be varied during the process (referred to as "light ramping"); and in others, the temperature may be ramped ("temperature ramping"). In each case, the result is a macrogranular porous CPVC product characterized by a high concentration ("conc") of chlorine (Cl) per unit of surface area ("Cl/unit area"), in chains near the surface of a macrogranule. Such a CPVC product is difficult to extrude or mold because of the relatively poor fusion characteristics of a mass of such granules.
Stated differently, the fusion temperature of CPVC from the photo-slurry process is higher than it would be if the Cl content was the same, but the Cl/unit area was lower. All reference to "Cl content" herein refers to chlorine chemically bound in the polymer chains of the resin. Granules which have a desirable Cl content above 69%, and so distributed as to allow them to be easily fused are referred to as "being essentially uniformly chlorinated".
Further, despite the advantages of the photochlorination (or "photo-slurry") process in which UV-light provides initiation of the reaction, there are conspicuous disadvantages. Among the main advantages are: excellent heat transfer; no catalyst residue in the product because no catalyst is used; and no residue of any diluent or swelling agent because neither is used. Among the main disadvantages are: a small reaction zone circumscribed by the depth of penetration of the UV-radiation; relatively very long reaction time for obtaining a Cl content of more than 67% in the CPVC product.
Numerous processes, other than photochlorination, have been proposed, but the predominant commercial photo-slurry process is disclosed in U.S. Pat. No. 4,412,898 to Olson et al. Except for the liquid chlorination processes, such as one disclosed in U.S. Pat. No. 4,350,798 to Parker, we are unaware of any process which does not produce CPVC having a high Cl conc near the surface. We know of no prior art process which yields 72% Cl CPVC which is fusible so as to provide a continuous phase at 170.degree. C.
The goal of the present invention is to provide a controllable organic peroxide or peroxyester catalyzed (all such compounds with a peroxy linkage are hereafter referred to as "peroxy compounds" and the reaction as being "peroxy-catalyzed), non-photochlorination process to chlorinate PVC homopolymer in an aqueous slurry, and to make a CPVC macrogranular product with a Cl content of at least 70% by weight (% by wt, or "% Cl") with a lower concentration of chlorine near its surface than can be obtained with any commercial process; and, to do so in a commercial reactor in which reactor productivity is at least doubled, compared to that of a conventional reactor used in the photo-slurry process. Most of all, such CPVC is to be made without sacrificing product quality.
We reached the goal with a two-step process; in the first step, PVC is chlorinated with molecular chlorine until it contains from 67-72% by wt Cl, at a relatively low temperature in the presence of an organic peroxy compound; and in the second step, the Cl content of the CPVC produced in the first step ("first step CPVC") is then increased by at least 3% to within the range from 70% to about 75% Cl, within a surprisingly short time, not more than about 3 hr even for 75% Cl content, by chlorination at a relatively higher temperature than that used in the first step, preferably in the presence of from 1-100 ppm of molecular oxygen and/or additional peroxy catalyst which may be the same as, or different from that used in the first step.
The temperature and time factors which affect the activity of organic peroxy catalysts which are short-lived under even highly favorable conditions, the general disinclination to use such catalysts and solvents therefor, both of which remain in the product, and the inherent sensitivity of a highly exothermic free-radical chlorination reaction, militated against the choice of an organic peroxide-catalyzed water-chlorination reaction.
To avoid photochlorination and the use of a swelling agent, Ackerman et al in U.S. Pat. No. 4,386,189 disclosed a high-pressure process in which they chlorinated PVC resin in aqueous 10-30% hydrochloric (HCl) acid in the presence of a sufficiently large excess of liquid chlorine to form a distinct liquid chlorine phase, and catalyzed the reaction with a solution of organic peroxy compounds.
In greater detail, Ackerman et al suspended granules of PVC (57% by wt Cl) in 24% HCl acid, and rid the suspension of oxygen by sparging nitrogen through it. Gaseous chlorine was then sparged through the suspension to drive off the nitrogen. A large excess of liquid chlorine was then slowly added to the suspension at room temperature which was mixed to ensure thorough contact between chlorine and PVC resin to ensure the presence of a liquid Cl phase. To this suspension was then added a mixture of organic peroxy compounds dissolved in appropriate solvents. The suspension was heated until the temperature of the suspension began to rise to a maximum of 75.degree. C., whereupon the temperature was modulated to stay in the range from 55.degree. C.-70.degree. C. Upon obtaining the desired Cl content in the CPVC produced, remaining excess chlorine was vented, and residual chlorine (with the product) is removed by sparging nitrogen through the suspension. A slurry of the CPVC product was then filtered and dried. The CPVC recovered had a specific gravity of 1.575 indicating a Cl content (covalently bonded) of 67%.
The Ackerman et al process relied upon the use of a large excess of chlorine and aqueous 10-30% HCl the presence of which affected the reactivity of the peroxides they used. As a result they sought to limit the temperature of their reaction to about 75.degree. C. To maintain a desirably low temperature, they must remove the heat of reaction, as evidenced by the large difference between the reaction temperature and the temperature of the water in the jacket (see Example 1 where the reaction temperature is above 75.degree. C. but the jacket temperature is 10.degree. C.). The limitations of the Ackerman et al process dictate that their chlorinated resins contain from about 62-69% by weight chlorine.
In contrast, our process is specifically directed to making CPVC resin having a chlorine content of at least 70%, typically about 72-73%. Yet to make a Cl content in the range from 70-75%, we prefer to use no more than a slight excess, less than 10 % by wt over stoichiometric of the Cl required to yield the desired end product; we add no HCI, and carry out the bulk of the reaction at a temperature below 90.degree. C., the second step of our process being run at from 90.degree. C.-130.degree. C.
The novel process relies mainly upon the unexpected effectiveness of certain organic peroxy compounds in an acidic environment essentially free of liquid chlorine. It is unnecessary to remove a large amount of heat, the jacket temperature being the same as, or higher than that of the reaction mass during most of the reaction, so that a portion of the reaction proceeds substantially adiabatically. In other words, we may start with a cool reaction, remove no heat during the remainder of the first step of the reaction, and heat during the second step, to keep the temperature of the reaction mass desirably high.
The significance of Ackerman et al carrying out the reaction in the presence of an excess of chlorine and a large amount of aqueous (say 10-24% HCl) cannot be overlooked because the concentration of peroxide catalyst is typically very low, in the range from 10.sup.-4 to 10.sup.-5 mole/liter and is affected by the concentration of hypochlorous acid (HOCl) in the slurry being chlorinated. When the slurry contains 10.9% HCl (the formality of electrolyte is 3.987) the concentration of HOCl is 1.1.times.10.sup.-6 mole/liter; when the slurry contains 18.8% HCl the concentration of HOCl is only 6.2.times.10.sup.-7 mole/liter (see "The Solubility of Chlorine in Aqueous Solutions of Chlorides and Free Energy of Trichloride Ion" by M. S. Sherril and E. F. Izard, Research Laboratory of Physical Chemistry, Massachussets Institute of Technology, J. Am. Chem. Soc., Vol 53, pg 1667, May 1931). Without the addition of HCl to the system we have less than 0.5% HCl, typically from 0.05 to 0.1 moles/liter of HCl (about 0.18% to 0.36% HCl) equivalent to 0.05 to 0.1 moles/liter of HOCl. This difference in HOCl concentration between the Ackerman et al system and that of this invention, is more than 10 times (one order of magnitude); it is more than three orders of magnitude (10.sup.3) different.
In an earlier post-chlorination process described in U.S. Pat. No. 3,632,848 to Young et al, PVC is chlorinated in an aqueous suspension at a temperature above 100.degree. C. to eliminate the induction time typically required in a slurry process, generally, and to help drive all oxygen from the suspension. However, at a temperature of 100.degree. C. the relative partial pressure of chlorine is so low that there is less than 3% chlorine in the suspension. At such low concentration, despite the reaction being carried out above the glass transition temperature (T.sub.g) of the PVC resin, there is so little CPVC formed in each granule, that even if all the chlorine is immediately reacted, the dense CPVC formed on and within each granule does not initially slow down the rate of chlorination. However, as the reaction proceeds, enough dense CPVC is generated to form an increasingly effective barrier to infiltration of Cl radicals, and both the rate of chlorination and the concentration of Cl in the macrogranules begin to reach a limit. As stated in the '848 patent, after 139 minutes, Cl conc reaches 68.4% Cl in the final product.
This blocking effect of Cl introduced near the surface is more readily appreciated by reference to the appended FIG. 1 in which several curves, each for a different process are presented.
Curve `D` graphically represents the % Cl in the CPVC as a function of time of reaction, for the runs in Table II of the Young '848 patent. Though the curve `D` is initially steep, as the %Cl increases, the denseness of the CPVC near the surface of each granule greatly impedes access of additional Cl. radicals, and the curve flattens out. Clearly it will take very much longer to reach 70% Cl.
It will be noted that Ackerman provides no data for the initial portion of the curve, from 57% Cl (see example 1 col 8, line 50, which is a typical Cl content of commercial PVC) to his first point, 65% Cl after 75 min (see his example 4). Therefore the curve `A` is smoothed to the 57% point.
In sharp contrast, the effect of our two-step process is graphically illustrated in a representative curve, identified as `B` in FIG. 1.
There are several advantages of our two-step process: (1) it does not require the use of added aqueous HCl; this obviates the necessity to recover, purify and recycle the HCl; (2) it does not require the use of a large excess of chlorine, which obviates the necessity to recover, purify and recycle the chlorine; and, (3) the reaction proceeds with a controlled rate of generation of heat which requires little, if any heat removal initially, then proceeds at elevated temperature in excess of 100.degree. C. in the second step. These features make a commercial reactor a practical reality. Further, not only is the product CPVC of excellent quality, but its high conc of Cl, in the range from 70-75%, is distributed throughout the granules so that the concentration of Cl near the surface is lower than it would be in any known, commercial aqueous chlorination process.